The present invention relates to a method and a device for determining partial discharges at an electrical component, which method and device can be used, in particular, in performing partial discharge measurements of electrical components such as, for example, high-voltage cables, rotating machines, transformers or the like.
A partial discharge measurement is a globally recognized method for quality control, both in the laboratory and in the field. Partial discharges are defined according to IEC 60270 as localized dielectric breakdowns of a small portion of a solid or liquid electrical insulation system under high voltage stress. In many cases, partial discharges indicate insulation defects at high-voltage components. Reliable recognition and monitoring of partial discharges protects against cost-intensive failures and repairs, and is therefore critically important. The partial discharge measurement is one of the main criteria in the evaluation of the quality of a cable or a set of cable fittings, and of their on-site installation.
Cables themselves are tested, for example, by means of a sensitive partial discharge measurement. During their production, the cables are tested in a shielded laboratory. For this purpose, the cable is subjected to a high voltage, for example an a.c. operating voltage of the cable, and a partial discharge measuring device is used to measure the partial discharges over a predefined period of time. Owing to the differing causes of partial discharges such as, for example, air inclusions in the insulation, in the case of the applied a.c. voltage partial discharges occur at differing times in the course of a period of the a.c. voltage. Each partial discharge causes a current pulse or current surge, which is converted, by means of a coupling capacitor and a measuring impedance, into a corresponding voltage pulse. This voltage pulse is then processed further to determine the partial discharge. FIG. 1a shows the waveform of such a voltage pulse in the time domain, and FIG. 1b shows the frequency spectrum of the voltage pulse of FIG. 1a. The voltage pulse shown in FIG. 1 is an almost ideal voltage pulse from a partial discharge calibration device. Since, however, the partial discharge occurs in a cable or in a comparable test object such as, for example, an electrical machine or a transformer, and the voltage signal can only be measured outside the test object, the voltage signal undergoes low-pass filtering before it can be measured, owing to the electrical properties of the test object. FIG. 2a shows a low-pass-filtered voltage pulse signal, and FIG. 2b shows the corresponding frequency spectrum. However, real test objects such as, for example, high-voltage cables or other electrical machines give rise to very complex filterings of the partial discharge signal. An example of a real partial discharge signal of a real test object is represented in FIG. 3a, and FIG. 3b shows the corresponding frequency spectrum of the partial discharge signal.
As mentioned previously, the partial discharges in the test object can be converted into a corresponding voltage signal, for example by means of a coupling capacitor and a measuring impedance. For the purpose of determining the partial discharge of the test object, this voltage signal, as shown in FIG. 4, is amplified by means of an amplifier 1 and filtered by means of a bandpass filter 2. The filter usually has a variable upper and lower cut-off frequency. The filtering is usually set to a frequency band having a low disturbance level, in which the broadband partial discharge pulse contrasts clearly with the background noise. The filter can be, for example, a bandpass filter having a passband range from 50 to 350 kHz. The low-pass characteristic of the filter provides for integration of the pulse waveform, such that a charge-proportional signal 3 is obtained. The charge-proportional signal 3 is supplied to a processing unit 4, which collects the charge-proportional signals obtained in a predefined period of time as a result of a plurality of partial discharges, and represents them, for example in the manner represented in FIG. 4, over the voltage applied to the test object. FIG. 5 shows this representation in detail. A complete phase cycle of the voltage applied to the test object is represented as a graph 5 over time. In the present example, the a.c. voltage applied to the test object has a frequency of approximately 60 Hz, such that a complete cycle takes approximately 16.67 ms. The test object is subjected to the test voltage, indicated by the graph 5, for a predefined period of time, being 47.39 s in the present example. Very many partial discharges, which are each entered as a dot 6 in the diagram represented in FIG. 5, occur in the test period. A partial discharge is entered in the diagram in respect of its phase position in relation to the test voltage 5 and the magnitude of its charge. If there is a greater accumulation of points 6 at a location in the diagram, this accumulation can be marked, for example, through use of a different colour for the points 6, in dependence on the frequency. A cause of the partial discharges can be inferred from the phase position, magnitude of charge, charge sign and the frequency. Finally, the processing unit 4 determines an overall charge of the partial discharges over the predefined period of time, which overall charge indicates the quality of the test object, for example of a high-voltage cable of predefined length.
In the case of the previously described method for determining the partial discharges in a test object, various problems can occur, which can falsify the partial discharge measurement result. For example, external disturbance quantities such as, for example, electromagnetic radio waves, can produce signals in the test object that are erroneously evaluated as a partial discharge pulse. It is usually attempted to eliminate such disturbance sources by operating the test object and the measuring device in a shielded room. This is very demanding of resources, however, particularly in the case of large test objects such as, for example, electrical machines or transformers. Furthermore, it is attempted to prevent such disturbances through use of an appropriate filter 2. In this case, however, there is the risk of actual partial discharge signals also being filtered out, in addition to the disturbance signals.
A further problem in partial discharge measurement is the separation of a plurality of superposed partial discharge sources from one another. In the case of the superposition of a plurality of partial discharge sources and/or of stochastic pulse-type disturbance sources, a superposed signal, or pulse mixture, which does not afford precise information concerning the nature and intensity of individual partial discharge sources in the test object examined, is produced at the output of the filter 2. The informative quality of the partial discharge measurement result can be diminished as a result. A further, specific problem in partial discharge measurement, particularly on cables, is that of so-called negative superposition. In this case, a partial discharge pulse reflected at the cable end becomes superposed on an original partial discharge pulse. In the case of certain cable lengths, this, in combination with filter natural oscillations and/or zeroizing of the bandpass filter 2, results in a negative superposition, or even extinction, as a result of which a partial discharge fault on the cable can be overlooked. In order to preclude such a negative superposition behaviour, partial discharge measuring devices are usually tested by double pulse calibrators. In this case, two successive pulses of defined charge are input, at a variable and defined interval, into the measuring system, wherein the display reading may only deviate downwards by less than 10% from the charge value of the pulse. This requires elaborate bandpass filters 2 that pass these tests, and may possibly result in bandpass limitations of the bandpass filter 2.